Combine mixamo animations with own rig / animation possible?
You have to keyframe the influence of IK (or other controls from your rig- for example, I have my hand and foot bones copy transforms from my respective hand and foot controllers) down to 0 for the Mixamo actions. Just go to every single Mixamo animation at the frame 0 and keyframe every custom influence/bone constraint to 0. You also have to do the reverse for your custom animations.
This approach works perfectly for exporting animations to Unity. I don't know how you're going to be using NLA editor so I can't say if it's going to work for you.